1. Field of the Invention
This invention relates to a radio card for allowing data transfer to be effected to and from a radio reader/writer in radio communication or allowing power reception from the radio reader/writer.
2. Description of the Related Art
In recent years, with the spread of radio cards, there has been a growing demand for a plurality of radio cards (hereinbelow referred to an unit radio cards) to be used in an overlapped state. When such unit radio cards are used in the overlapped state, there arises a problem such as a lowering in reception power resulting from antenna-to-antenna coupling in respective unit radio cards.
In a conventional radio card, however, the greater the aperture area of a coil (loop), the larger the reception power becomes. For this reason, use is often made of a loop antenna which can achieve the greatest aperture area in accordance with the outer configuration of the radio card. In a radio card of a rectangular configuration of a standard size, for example, use is often made of a loop antenna of a rectangular configuration corresponding to the outer configuration of the radio card so as to obtain the greatest loop.
In such a radio card, if a plurality of unit radio cards are placed one over the other, then the unit radio cards have their antennas substantially matched in configuration to each other. Therefore, their antenna-to-antenna coupling becomes stronger and there arises a lowering in reception power in the respective unit radio cards, thus exerting an adverse effect on the communication distance involved.
In order to solve such a problem, there is a conventional technique according to which, upon placing two unit radio cards one over the other in a face-to-face relation, their antennas are made not to be matched in configuration to each other. In this technique, it is assumed that an antenna is formed on the surface of the radio card and, based on this, the unit radio cards are made not to be matched in configuration to each other in a mutually opposite state.
Since, however, a normal radio card is often made of a thin card-like body, if the antennas of the unit radio cards are matched in configuration to each other even without the unit radio cards being set in a face-to-face relation, then the resonant frequency varies and there arise a lowering in reception voltage and a shortening in communication distance involved.